Fins for eliminating backstreaming in a vacuum pump



' US. Cl. 230-401 United States Patent 3,454,214 FINS FOR ELIMINATING BACKSTREAMING IN A VACUUM PUMP Norman Milleron, Berkeley, Calif., assignor to the United States of America as represented by the United States Atomic Energy Commission Filed Oct. 25, 1967, Ser. No. 678,116 Int. Cl. F04f 9/00; F15d 1/08 2 Claims ABSTRACT OF THE DISCLOSURE A screen for use in diffusion type vacuum pumps to prevent pumping fluid vapor backstreaming from the mainstream of the jet from entering the region being evacuated. Louvered metal filters are disposed adjacent the jets to intercept the backstreaming pumping vapor.

Background of the invention This invention relates generally to vacuum pumps and more particularly to a means for stopping the backstreaming of vapor from the jets in a diifusion pump toward the enclosure being evacuated. The invention described herein was made in the course of, or under Contract W-7405-ENG-48 with the Atomic Energy .Commission.

Backstreaming is one of the more bothersome problems encountered when using vacuum pumps. Backstreaming vapor from the pumping fluid, usually oil, can contaminate the vacuum enclosure and cause serious problems, as for instance'when high voltage apparatus is to be operated within the vacuum enclosure or when high purity ion sources are operated. conventionally such backstreaming is reduced by providing relatively large baifles between the vacuum pump and the vacuum enclosure, but a cold trap is still necessary to condense the remaining vapor.

Backstreaming is usually particularly bothersome at the start of pumping when the region to be evacuated is at or near atmospheric pressure. The backstreaming oil vapor is scattered by collision with the many gas particles present at such time and is thus deflected around baifles, which are mainly effective only for oil vapor traveling in a straight line from the jet. Cold traps must be included to capture the oil vapor which passes by the baflies.

A further problem with conventional baflles is the pressure created in the space between the jets and the baflies by the backstreaming oil vapor. Such oil vapor interferes with gas particles drifting from the region being evacuated to the jets and thereby retards rapid and complete evacuation of the region.

Summary of the invention In the present invention a fine louvered screen, disposed between the pump and the vacuum enclosure, intercepts the backstreaming vapor. Such screen has a sloping configuration for automatically returning intercepted oil to the pump reservoir, thereby avoiding clogging the small openings in the screen.

Providing fine openings in the screen permits the placement of the screen immediately adjacent the top of the jet so that there is no appreciable space where the backflaring oil vapor can repel gas particles. Therefore, the gas particles can readily enter the jet and be trapped without encountering back pressure from backstreaming "ice oil vapor. Pumping speed and efficiency is thereby increased.

Furthermore, the placement of the bafile immediately adjacent the jet permits capture of the oil vapor before scattering can occur, and thus backstreaming at the start of pumpdown is suppressed.

It is an object of the present invention to provide a relatively simple and inexpensive means for reducing backstreaming in a diffusion type vacuum pump.

It is another object of the present invention to increase the pumping speed of a vacuum diffusion pump by reducing back pressure caused by backstreaming pumping fluid vapor.

Brief description of the drawing The invention will be best understood by reference-to the following description together with the drawing of which:

FIGURE 1 is a top view of a diffusion vacuum pump,

FIGURE 2 is a section view of a vacuum pump taken v ture taken at line 33 in FIGURE 2? Description of the preferred embodiment fluid 13 is provided at the bottom of the pump 11. A'

heating element 14 is disposed beneath the fluid 13, causing the fluid to vaporize and rise upwardly in a lower stack 16 generally having a hollow cylindrical configuration. A lower jet forming nozzle 19 has a truncated cone shape and is disposed above the upper end 21 of lower stack 16. An upper stack 22 leads pumping vapor upwardly from the top of lower nozzle 19 to an upper nozzle 23.

The pumping vapor ascends upwardly through the stack 22, a portion being formed into a jet by nozzle 19 and the remainder being ejected as a jet from nozzle 23. The general configuration and direction of a jet is indicated in FIGURE 2 by dashed lines 24, while the path of backstreaming vapor is indicated by arrows 26.

The backstreaming stopping means provided by the present invention is obtained by an annular lower louvered screen 27 and an annular upper louvered screen 28 encircling nozles 19 and 23 and extending outwardly from the nozzles to housing 12, sloping downwardly to- Ward the housing wall 12 so that condensed pumping fluid on the screens will flow by gravity to the wall and downwardly back to the reservoir 13. The general configuration of the screen 26 is that of a truncated cone, while screen 28 has an accordion-type fold with alternating peaks and valleys. Screen 27 encircles a nozzle cap 25 which fits over nozzle 23.

For the detailed structure of screen 28, reference is made to FIGURE 3 wherein long narrow louvers 31 in screen 28 are indicated, such louvers sloping downward- 1y with the long dimension extending radially outwardly from the nozzle cap 25 to the wall 12. Considering that the backstreaming vapor tends to travel in straight lines, the louvers are designed to prevent any line-of-sight path from the jets into the region being evacuated. For trapping oil vapor, the optimum spacing for the louvers 31 is approximately 0.060 inch as indicated by distance X in FIGURE 3. With narrower louver openings 32, condensed pumping fluids tend to bridge and close the gap between louvers. With wider openings there is more chance that the oil vapor will escape through the screen since there is more chance of collisions with gas particles which may deflect the vapor through one of the gaps. When the vapor 26 contacts the relatively cool screen 28, the oil vapor condenses on the screen and returns by gravity to the reservoir 13.

In use, it has been found that the backstrearning has been almost entirely eliminated, particularly at the usually bothersome times when pumping is just being started or is being stopped.

The screens 27 and 28 provide an advantage over conventional vapor traps in that the screens are disposed immediately adjacent the uppermost portion of the jet, thereby allowing the gas being evacuated to reach the jet while encountering minimal back pressure from backstreaming pumping fluid vapor. Thus the pumping speed of the present pump may exceed that of the previous pump by 25 to 50 percent at high vacuum conditions.

While the lower screen 27 may be constructed in the same manner as the upper screen 28, generally a trun-- cated cone with radial louvers will function satisfactorily for the lower screen 27 since complete elimination of backstreaming is not so important at the lower jet.

Many variations are possible in the design. For instance, the number of jet stages provided may be varied. Only a single jet with the accordion-folded screen might be used or, on the other hand, three jets might be provided. Also, other pumping fluids might be substituted, although oil is preferred with the particular design described:

Therefore, it is not intended to limit the invention except as defined in the following claims.

What is claimed is:

1. A vacuum pump of the class having an annular outer wall and a reservoir for pumping fluid at the base thereof, such fluid being heated and directed to at least one jet forming nozzle to provide a jet of vapor for entrapment and removal of gas particles from a region being evacuated, the combination comprising an annular screen disposed immediately adjacent a jet formed from said nozzle and between said jet and said region to be evacuated, said screen being a heat conducting material having louvers formed therein, the long dimension of said louvers extending radially outwardly from said nozzle toward said wall, said screen being sloped down- 4 wardly from said nozzle to said wall whereby pumping fluid condensed thereon can flow by gravity to said wall, and said screen having an accordion-type fold with alternate peaks and valleys extending outwardly from said nozzle to said wall and parallel to louvers in said screen immediately adjacent thereto.

2. A vacuum pump according to claim 1 wherein there are at least two said jet forming nozzles disposed such that a first of said nozzles is nearest said region being evacuated, wherein said screen having an accordion-type fold therein is disposed adjacent said first nozle, and wherein there is associated with at least one remaining 1 jet an annular screen disposed immediately adjacent that jet and between that jet and said region to be evacuated, said screen being a heat conducting material having louvers formed therein, the long dimension of said louvers extending outward from said nozzle toward said wall, said screen being sloped downwardly from said nozzle to said wall whereby pumping fluid condensed thereon can flow by gravity to said wall, and the latter said screen has a truncated cone configuration.

References Cited DONLEY J. STOCKING, Primary Examiner. W. J. KRAUSS, Assistant Examiner.

US. Cl. X.R. 138-42 

